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Summary Understanding organic porosity and its structural development in source rock reservoirs is essential to understanding how it can influence flow properties. A field emission scanning electron microscope (FESEM) was used to study the structure of the organic matter (OM) in shale samples as maturity increases. Argon ion milling of shale samples has proven to be a very powerful tool in understanding pore systems in shale, however, artifacts from this technique have been shown to obscure the OM structure. Consequently, fresh cleavage samples were imaged in addition to the argon ion-milled samples. The presence of oil/bitumen creates a challenge to observe the OM pore system by SEM techniques. To overcome this problem eight shale samples, from six different geological formations with a maturity range from 0.66 to 1.82 %Ro equivalent were observed, before and after CO2-toluene cleaning. OM at low maturity levels (<0.7 %Ro equivalent, from T-max values) is composed of sub-spherical units, generally 7–12 nm in diameter. With increasing maturity, these spherical subunits are connected, creating a network of OM. The spaces between these particles and the spaces within the connected framework determine the OM pore sizes, shapes, and distribution. Observations made by SEM showed OM structural changes from spherical structures to a crosslinked network in the OM that may be associated with maturity. Introduction Porosity occurring within the OM of potentially oil producing shales is not as well understood as in gas producing shale plays. Understanding the OM structure in oil plays may help understand flow and porosity measurements and help resolve challenges in establishing a multiscale approach. Coring is defined as the downhole acquisition and recovery of reservoir formation material, so it is important to understand that all laboratory testing is conducted on samples in as received (AR) state which may not be in their unaltered in situ state prior to core retrieval, and it is likely that some changes have occurred during coring, sampling, and handling procedures (Handwerger et.al. 2012). Observing pore systems in oil producing plays is difficult as the pore spaces may be filled or partially filled by fluids (indigenous and/or coring fluids) which makes it difficult to distinguish OM structures by SEM techniques. For this reason, observation of shale samples needs to be performed on AR samples as well as after cleaning to better observe changes in the pore system. There are several laboratory techniques available to clean core samples and, in general, they all have positive and negative attributes. The selection of best solvents greatly depends on rock type, the ability to remove fluids, and must not react with the rock sample. The CO2-toluene (CO2-tol) core cleaner is a widely used apparatus to clean crude oil, water, and drilling mud liquids from whole core samples in preparation for porosity and permeability measurements and was used in this study for the after cleaning SEM observations. While the removal of pore filling material will increase porosity, shrinkage of the OM may also take place thus increasing porosity. In 2010 Loucks et.al. described a type of pores that appeared to be the result of OM shrinkage. SEM observation of CO2-tol cleaned samples helped to understand to what extent the OM had undergone shrinkage compared to the AR samples when using this method.
Summary Calcite forms variable proportions of source-rock reservoirs ("shale plays"). Although calcite content can be quantified via petrophysical analyses, XRD, XRF and other techniques, the amount of calcite, by itself, is not enough information to predict the likely importance of these minerals for reservoir and completions quality. Four principle types of calcite can be recognized:Pelagic components, mostly foraminifera and coccoliths, form a large component of the Eagle Ford and Niobrara but other types of pelagic carbonates (e.g., tentaculitids) are common in Paleozoic source-rock plays such as the Marcellus, Carbonate "event beds" (turbidites, storm deposits, etc.) are present in the Avalon, Barnett, Vaca Muerta and other plays, In situ benthic carbonates (bivalves, corals) are present in some plays (e.g., Eagle Ford, Marcellus), and Diagenetic calcites (pore filling cements, fracture fills, replacements, etc.) are present to varying degrees in perhaps most source-rock plays. Detailed core descriptions and petrographic observations are critical for assessing the origin of the calcite. Similar concepts apply to other mineral and organic components of mudstones.